Generally, organic semiconductor devices including organic diode devices and organic transistor devices are based on the electrical semi-conductivity that relates to the HOMO (Highest Occupied Molecular Orbital) energy level and the LUMO (Lowest Unoccupied Molecular Orbital) energy level of organic materials. Examples of the organic diode devices include organic light emitting diodes and organic electroluminescent (EL) diodes, and examples of the organic transistor devices include organic FET (Field Effect Transistors), organic TFT (Thin Film Transistors), organic SIT (Static Induction Transistors), organic top gate SIT, organic triodes, organic grid transistors, organic thyristors, and organic bipolar transistors. In these organic semiconductor devices, the electrical and optical properties of the devices are strongly depending on the thin film structure of the organic layers formed on a substrate. Thus, the development of the thin film having an efficient structure is technically as important as the development of new organic materials. The present invention relates to a method for manufacturing an organic semiconductor thin film having a new structure with high efficiency, and a device including the organic semiconductor thin film. The present invention can be widely applied to the above-mentioned various organic semiconductor devices.
Hereinafter, the present invention will be described with reference to the organic EL device, which has the simplest structure among the above-mentioned devices. In the organic EL device, a thin film including fluorescent organic compounds is positioned between electrodes, cathode and anode. When a driving voltage is applied to the organic EL device, electrons and holes are injected into the LUMO and HOMO levels of the fluorescent organic compound of the thin film from the cathode and anode, respectively, and the injected electrons and holes are recombined to produce excitons, which emit light (fluorescence or phosphorescence) through losing their activity. In the present invention, a light-emitting device represents an image display device using the organic EL device. The light-emitting devices also include the following modules: a module formed by mounting a connector such as an anisotropic conductive film, FPC (Flexible Printed Circuit), TAB (tape automated bonding) tape or TCP (Tape Carrier Package) to the EL device, a module where a printed circuit board is installed at the end of the TAB tape or TCP, and a module formed by mounting an IC directly on the EL device by a method of COG (chip on glass). Recently, a method for producing an organic or inorganic semiconductor device on a substrate has been considerably developed, and an active matrix display device (light-emitting device) including the organic or inorganic semiconductor device has been also being developed. In the present invention, the semiconductor device also represents a single device or a plurality of devices, which have a switching function.
The organic EL device (also referred as ‘EL display device’) is a self light-emitting device, and can be produced as a simple passive matrix light-emitting device or an active matrix light-emitting device using TFT. In the organic EL device, organic EL layers are positioned between electrodes, as shown in FIG. 1a. As shown in the figure, the organic EL layers generally have a multi-layered structure, in which the boundary or interface of each layer is clearly distinguished. The representative example of the multi-layer structure, suggested by Tang, et al., includes a hole transporting layer 13, a light-emitting layer 14 and an electron transporting layer 15 (Tang. C. et al. Appl. Phys. Lett. 1987, 51, 913-915). This structure shows high light-emitting efficiency and thus the structure is adopted in almost all kinds of EL devices. Another examples of the multi-layer structure include a structure having a hole injecting layer 12, a hole transporting layer 13, a light-emitting layer 14 and an electron transporting layer 15 which are sequentially formed on an anode 11 of a substrate 10, and a structure having a hole injecting layer 12, a hole transporting layer 13, a light-emitting layer 14, an electron transporting layer 15 and an electron injecting layer 16 which are sequentially formed on an anode 11 of a substrate 10 (See FIG. 1a). The light-emitting layer 14 can be doped with fluorescent dopants. Besides the monomeric low molecular weight EL material, conjugated polymers such as poly(phenylvinylene) were introduced as the EL material in 1990 by Burroughes et al. (Burroughes, J. H. Nature 1990. 347. 539-541). Recently, stability, efficiency and durability of the polymer EL material have been remarkably improved. In this specification, all layers sandwiched between the electrodes are called as the EL layers. Accordingly, the EL layer 20 includes the hole injecting layer 12, the hole transporting layer 13, the light-emitting layer 14, the electron transporting layer 15 and the electron injecting layer 16. When a voltage is applied to the EL layer 20 from the electrodes 11, 17, the electron-hole are recombined at the light-emitting layer 14 to induce the light-emission. In this specification, the EL device also represents the light-emitting device including the electrodes 11, 17 and the EL layer 20. In order to prevent the EL device from being deteriorated due to the exterior air, the substrate (EL panel) on which the EL device has been formed is encapsulated with a sealing material (packaging), and is bonded to a cover member. Then, the connectors (FPC, TAB, etc.) are mounted for connecting the encapsulated EL device to an external driving circuit, which produces a passive or active matrix light-emitting device.
The EL layer 20 can be formed by various methods. Exemplary methods include dry processes such as vacuum evaporation and sputtering, and wet processes such as spin coating, a cast method, an inkjet method, a dipping method, and a printing method. Besides, roll coating, an LB method and ion plating method can also be used. In case of using a low molecular weight compound having a good thermal stability and capable of being sublimated to form a thin film, the dry process such as vacuum evaporation is generally used to manufacture the multi-layer EL device shown in FIG. 1a. However, the dry process requires a high vacuum environment, the manufacturing conditions should be controlled carefully, and thus the process for fabricating EL devices is complex, resulting in the large manufacturing costs. On the contrary, the wet process, which uses a solution of the organic compounds dissolved in a solvent, and forms an organic layer of the dissolved compounds, has an advantage in that the EL layer can be easily formed. Moreover, the wet process can be used not only for the low molecular weight compound but also for the organic polymer materials. On the other hand, there are disadvantages in the wet process, because of the solvent problem for forming multi-layer structure. Namely, different solvent must be used to form each different layer in forming the multi-EL layer. In this case, to form an upper organic layer on the lower organic layer, a solution, which does not dissolve the lower organic layer formed previously on the substrate, must be used to form the multi-EL layer. However, it is generally difficult to select appropriate set of the solvents to form the multi-EL layers. If an improper solvent is used, the organic layer previously formed on the substrate 10 may be re-dissolved by the solvent to form a new upper layer, and some of the organic materials of the lower or upper organic layers may irregularly diffuses into the neighboring layers. Thus, it is difficult to manufacture the multi-EL layers 20 of FIG. 1a by simply repeating the conventional wet process. Therefore, as shown in FIG. 1b, the wet process is generally carried out to form a single-EL layer 20 in which one or more compounds are uniformly dispersed in the single-EL layer 20. As a result, the EL device manufactured by the conventional wet process shows the low light-emitting efficiency and requires high driving voltage. In some cases, the multi-EL layer 20 can be formed by combination of the wet process and the dry process. However, the multi-EL layer 20 produced with this method also has the low light-emitting efficiency and requires high driving voltage.